Porcelain dental materials such as jacket crowns, inlays, veneer on metal and denture teeth are generally prepared by adding distilled water to raw porcelain and then conducting the two operations of condensation and densification. Condensation is herein defined as the process of green-biscuit preparation. Densification is defined as those pyro-bonding processes that form the mature porcelain.
Condensation is acheived by pressing or packing the porcelain material into the desired shape. Densification is accomplished by firing the condensed porcelain to an appropriate temperature. Depending upon the strength of the green-biscuit the shape may be manipulated either before or after firing by such techniques as machining.
Shrinkage and distortion of the material (green-biscuit) often occur upon firing. Additives have been tried in the wetting stage to develop green strength. However, no additives have been formulated specifically to prevent deformation in firing, particularly that known as "balling" wherein sharp edges become rounded off during firing. It is noteworthy that other properties of both the green-biscuit and the fired or final product which are necessary in order to have an acceptable dental product are adversely affected by all known additives.
For example, organic silicates, particularly tetraethyl silicate and sodium metasilicate have been recommended for use with dental porcelain in British Pat. No. 499,959. However, it has been found that the use of organic silicates for dental porcelain is undesirable because in the amounts required to restrict "balling", the green-body bulk density is too low. This poor initial packing causes excessive firing shrinkage. Also, in vacuum or reducing or inert atmospheric fires, char will be left behind. Furthermore, sodium metasilicate cannot be used because it does not satisfactorily restrict "balling" and causes severe and excessive opacification.